tetrachord
noun
tet·ra·chord ˈte-trə-ˌkȯrd 
: a diatonic series of four tones with an interval of a perfect fourth between the first and last
Word History
Etymology
Greek tetrachordon, from neuter of tetrachordos of four strings, from tetra- + chordē string — more at yarn
First Known Use
1603, in the meaning defined above